Bridging the Gap between Solar Cells and Batteries: Optical Design of Bifunctional Solar Batteries Based on 2D Carbon Nitrides

While solar cell technology is booming, intermittent availability of sunlight motivates new vistas for multifunctional devices capable of energy capture and storage on the same material, i.e., direct or two‐electrode bifunctional solar batteries. Herein, simulations and experiments are utilized to take a closer look at efficiency limitations and design considerations, and guidelines are proposed to operate a solar battery comprised of the 2D carbon nitride potassium poly(heptazine imide), K‐PHI, as a bifunctional solar battery photoanode in conjunction with the separator poly(N‐vinylcarbazole) and cathode poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate. An optical design of this device is developed by proposing light absorption in a charge collection layer within the photoanode and calculating photocharging current and charging time as figures of merit. The much larger efficiency of operation via rear illumination for K‐PHI layer thicknesses >200 nm is highlighted and enhancement strategies without modifying the photoactive layer are proposed. Finally, adapted Ragone plots are introduced and it is shown how the solar batteries are capable of improving energy and charge output solely via illumination (for the design under 1 sun, the energy and charge output increase by 60% and 63%, respectively) without modifying the device. A solar battery based on potassium poly(heptazine imide) as bifunctional photoanode by means of optical simulations and electrochemical measurements is designed. Performance improvement is modeled via Ragone plots and general design guidelines for solar batteries are put forward, including new approaches to circumvent the conceptual problem of active layer thickness.